1. Field of the Invention
The present invention relates to an apparatus and method for estimating an uplink frequency offset in a wireless communication system, and more particularly, to an apparatus and method for estimating an uplink frequency offset by using a random access channel.
2. Description of the Related Art
Recently, research on 4th generation systems, also referred to as next generation wireless communication systems, is being conducted in order to provide users with various services at a high transmission speed (about 100 Mbps or higher). The provision of a high speed service requires various techniques for a physical layer or a further higher layer. For high speed data transmission, the physical layer employs a multiplexing technique whereby one line (a pair consisting of a transmitter and a receiver in the case of wireless communication) is divided to obtain a plurality of communication paths (hereinafter ‘channel’) through which discrete and independent signals can be simultaneously transmitted and received. Examples of representative multiplexing techniques include a Frequency Division Multiplexing (FDM) technique and a Time Division Multiplexing (TDM) technique. Regarding the FDM technique, an Orthogonal Frequency Division Multiplexing (OFDM) method for high speed data transmission is being researched as a multiplexing technique for the physical layer in the 4th generation wireless communication system, which is currently being implemented.
In the OFDM method, data transmission is achieved by the use of a multi-carrier. This is a sort of Multi-Carrier Modulation (MCM) scheme in which transmission is made so that a symbol stream received in series is converted to a parallel symbol stream and then the symbol steam is modulated to a plurality of sub-carriers that are mutually orthogonal one another.
Since it is suitable for high data transmission, the OFDM method is adopted as a standard method in wireless communication such as an IEEE 802.11a system, high speed wireless local area network (LAN) of HIPERLAN/2, broadband wireless access (BWA) of an IEEE 802.16 system, and digital audio broadcasting (DAB). Furthermore, the OFDM method is also adopted as a standard method in wired communication such as asymmetric digital subscriber line (ADSL) and very high-data rate digital subscriber line (VDSL).
A frequency offset has a significant effect on the capability of a communication system employing the OFDM method. To solve this problem, a lot of researches have been conducted so far. However, most of the research is carried out under the assumption of a downlink condition where signal processing is carried out by a mobile station by receiving a signal that is broadcast from a base station and thereafter the signal is processed. Research on frequency offset estimation and compensation methods under the assumption of an uplink condition have not been sufficiently conducted.
Unlike downlink, in the case of uplink, several user signals are added when received. For this reason, the conventional frequency offset estimation and compensation methods which have been researched under the assumption of the downlink cannot be directly applied to the uplink. Therefore, there has been a theoretical approach for applying the conventional downlink frequency offset estimation method. In this approach, the same condition as the downlink is achieved when each user is allowed to use a different band, and a signal for each user is separated by providing a filter for each band. According to this approach, the number of filters for precisely separating each band is required to be as many as the number of users. Therefore, it is found that this approach is unrealistic in practice.
As described above, in the past, there was no technique for estimating the uplink frequency offset. In particular, since frequency synchronization is important in an OFDM-based system, the technique for estimating the uplink frequency offset is seriously demanded.